1. Field of the Invention
The present invention relates to a photoelectric encoder. In particular, the invention relates to improvements in a photoelectric encoder that has a telecentric optical system wherein a lens and an aperture are inserted between a main scale and a light receiving element.
2. Description of the Related Art
As described in Japanese Patent Laid-Open Publication No. 2004-264295 and as shown in FIG. 1, a photoelectric encoder is designed in which a lens optical system (telecentric optical system) 40, comprising a lens 42 and an aperture 44 that functions as a telecentric optical diaphragm, is inserted between a main scale 20 and a light receiving element array 34 constituting a light receiving unit 30, for example, and as shown in FIG. 2 this lens optical system can set the magnification by adjusting the distances a and b between the lens 42 and the scale 21 of the main scale 20 and between the lens 42 and the light receiving element 35 on the light receiving element array 34, respectively. In FIG. 1, the reference numeral 10 denotes a light source and the reference symbol f denotes a focal length of the lens 42.
In the photoelectric encoder that uses this telecentric optical system 40, an image on the main scale 20 is made pass through the lens optical system (42, 44) and is projected onto the light receiving element array 34. Here, by positioning the aperture 44 at the focal position of the lens 42, even when the distance (gap) between the main scale 20 and the lens 42 changes, fluctuations in the magnification of the image formed on the light receiving element array 34 can be controlled if the positional relationship between the lens 42, the aperture 44, and the light receiving element array 34 does not change.
In particular, when the lens array 46 is used as the lens 42 as shown in FIG. 3 (light path view) and FIG. 4 (perspective view), the entire size can be made smaller and the scale field of view (FOV) can be increased.
As shown in FIG. 3 and FIG. 5, however, when a photoelectric encoder employs the lens array 46, there was a problem in which the image was divided and reversed in each single lens optical system.
This is a problem with not only image patterns but is an especially serious problem with absolute types which are required to reproduce accurate shapes.